Absorbance detectors utilizing spectrophotometers are widely used as the detectors of liquid chromatographs. FIG. 7 is a schematic configuration diagram showing an optical system of a conventional absorbance detector disclosed in Japanese Unexamined Patent Application Publication No. 2002-202189 and other documents. A ray of light emitted from a deuterium lamp 10 (the light source) is directed onto a diffraction grating 14, which disperses the light over a range of wavelengths. From the light thus dispersed, a component of light with a specific wavelength is extracted through a slit 16 and delivered to a sample cell 17 containing a sample solution 18. For the light thus delivered, the intensity (or quantity) of light that has passed through the sample cell 17 is measured with a photodetector 19, such as a photodiode. An electric current thereby produced is converted into voltage signals by a current/voltage (I/V) conversion circuit section 20. These signals are further converted into digital data by an analogue/digital (A/D) converter 21 and sent to a data processor 22.
The absorption of light by the sample solution 18 causes the light quantity to be smaller than the value measured without absorption. The degree of attenuation depends on the concentrations of the components in the sample solution 18. Accordingly, it is possible to determine the quantities of the components in the sample solution by accurately determining the amount of absorbance, i.e. the magnitude of the change in the light quantity due to the presence of the sample solution 18. The wavelength of the measurement light (i.e. the light extracted through the slit 16 and delivered to the sample cell 17) depends on the angle of the diffraction grating 14, which can be rotated within a predetermined angular range by a grating driver 15 including a motor. Therefore, for example, it is possible to measure the intensity of the transmitted light while rotating the diffraction grating 14 in steps of small predetermined angles. The data obtained by this operation can be used to construct an absorption spectrum in the data processor 22.
The absorbance-based determination of the component quantities is premised on accurate measurements of the quantity of light both with absorption by the sample solution and without such absorption. However, the light-quantity data given to the data processor 22 may be affected by various factors as follows.
(1) Variation in the luminance of the deuterium lamp 10.
(2) Variation in the diffraction efficiency of the grating 14.
(3) Variation in the width of the slit 16.
(4) Variation in the size or transmittance of the light-transmission window of the sample cell 17.
(5) Variation in the photoelectric conversion efficiency of the photodetector 19.
(6) Variation in the conversion gain or other electrical characteristics in the I/V conversion circuit section 20.
(7) Read error of analogue signals in the A/D converter 21.
In view of these factors, the gain of the I/V conversion circuit section 20 in the conventional spectrophotometer is determined so that the light-quantity data (or the output of the A/D converter 21) measured without the absorption of light by the sample solution 18 will not saturate.
Among the various aforementioned factors, variations (3) to (7) are normally much lower than one percent, whereas the variations (1) and (2) can be as large as several tens of percent or more. This is due to the remarkable improvements in the luminous efficiency of deuterium lamps or the diffraction efficiency of gratings. Replacing the light source or diffraction grating in a user-owned absorbance detector with a new product having improved characteristics can result in a several tens of percent improvement in the light-quantity data obtained without absorption by the sample solution as compared to the data obtained before the part replacement.
In the worst-case scenario, the energy of light without absorption by the sample solution may exceed a previously estimated upper limit for the aforementioned reasons. If this situation occurs, the data processor 22 cannot correctly calculate the absorbance, so that it will provide inaccurate quantity-determination results. However, since there is no way of knowing the occurrence of the abnormal measurement, the user relying on the quantity-determination result provided by the data processor 22 will unintentionally prepare a wrong analysis report or provide the client with the unreliable measurement result.
The present invention has been developed to solve such a problem. Its first objective is to provide a spectrophotometer capable of detecting a situation in which the measurement cannot be correctly performed due to the replacement of the light source or diffraction grating or for other reasons, and then unmistakably informing a user of that situation to prevent the measurement from being incorrectly performed.
The second objective of the present invention is to provide a spectrophotometer capable of correctly calculating the absorbance even if the quantity of light reaching the photo detector exceptionally increases due to the replacement of the light source or diffraction grating or for other reasons.